Why Do Beavers Build Dam?

The beavers of North America build a dam to create an artificial lake in which to construct an island home or lodge. They are the second-largest rodent in the world (after the capybara). Their colonies create one or more dams to provide still, deep water to protect against predators, such as coyotes, wolves and bears, and to float food during winter and building material.

Beavers always work at night and are prolific builders, carrying mud and stones with their fore-paws and timber between their teeth. Because of this, destroying a beaver dam without removing the beavers is difficult, especially if the dam is downstream of an active lodge. Beavers can rebuild such primary dams overnight, though they may not defend secondary dams as vigorously. Beavers may create a series of dams along a river.

Often the beavers work in colonies. After choosing a narrow place in a shallow stream with a firm bottom, they set to work felling trees by standing on their hind legs and gnawing round the trunks with their large chisel-like teeth.

When the tree is down, the beavers lop off the branches and cut the trunk into suitable lengths which they drag into the stream and sink across the current. Sticks, stones and mud are used to keep the dam in position and make it watertight.

In the middle of the lake thus created the beavers use the same materials to build their lodge. When completed this is dome-shaped, ventilated structure about eight feet in diameter and rising well clear of the surface of the lake. There are two entrances, both under water. One of these is used for general purposes, the other as an escape route in an emergency or for bringing in food.

The lodge serves as a home, a nursery for the baby beavers and a storehouse for food in winter. The beavers feed chiefly on the bark of trees, of which they keep a plentiful supply at the bottom of the dam, on the bed of the lake and built into the fabric of their home. In winter, when the lake is frozen over and snow covers the ground, the lodge is virtually an impregnable fortress which the beavers can leave and enter by swimming under the ice.

Beavers are experts at keeping the water in the lake at the right level by constructing canals. They work industriously to maintain both dam and lodge-hence the origin of that popular phrase about being “as busy as a beaver”.

The North American beaver population was once more than 60 million, but as of 1988 was 6–12 million. This population decline is the result of extensive hunting for fur, for glands used as medicine and perfume, and because the beavers’ harvesting of trees and flooding of waterways may interfere with other land uses.

Content for this question contributed by Jill Pickerel, resident of Sebastopol, Sonoma County, California, USA